The invention relates to a projection exposure system, in particular for microlithography, for producing an image in an image plane of an object arranged in an object plane with a light source emitting a projection light bundle, with at least one projection optics arranged in an optical path between its object plane and its image plane and with at least one optical correction component arranged in said optical path in front of the image plane, which in order to change optical image properties in the projection is coupled to at least one correction manipulator in such a way that an optical surface of the optical correction component illuminated by the projection light bundle is moved at least regionally, wherein the correction manipulator cooperates with a correction sensor device in order to determine the optical image properties in the projection.
The invention furthermore relates to a process for compensating image defects occurring in the projection optics of a projection exposure system, in particular for microlithography.
A projection exposure system and a process of the type mentioned in the introduction are known from DE 198 24 030A1. There a wave front sensor is provided that is arranged in the region of the image plane. This sensor either operates in the exposure pauses or, in a manner that is not described, during the exposure. If the sensor is operated in the exposure pauses, then this reduces the throughput of the projection exposure system. If on the other hand such a sensor is employed during the exposure, the projection light absorbed by the sensor may not simultaneously serve for the projection exposure of the wafer. This reduces the lighting efficiency of the projection exposure system.
The object of the present invention is accordingly to develop a projection exposure system of the type mentioned in the introduction so that image defects can be corrected while at the same time achieving a high projection efficiency.
This object is achieved according to the invention in that the correction sensor device comprises:
a) a light source that emits at least one measuring light bundle that traverses at least a part of the projection optics and that lies in front of an entry to the projection optics and behind an exit from the projection optics outside the projection light bundle,
b) a position-sensitive correction sensor element for detecting the wave front of the at least one measuring light bundle.
The use of measuring light independently of the projection light ensures that the projection light can be used without loss for the lighting of the wafer.
Depending on the requirements placed on the accuracy of the determinations of the image defects, the greatest possible aperture region of the projection optics is traversed by the measuring light. Several measuring light bundles may also be used to provide a broader utilization of the aperture of the projection optics.
The generation of the measuring light bundle and of the projection light bundle may take place independently. For the measuring light there may therefore be used for example light of a specific wavelength that can be detected with a high degree of sensitivity using known sensors and that also does not interfere in the projection procedure if for example measuring light reflections reach the region of the image, Furthermore, all the projection light is available for the projection procedure.
Alternatively the measuring light bundle may be split off from the projection light bundle. As a rule a small proportion of the light output that is made available by the light source emitting the projection light is sufficient as measuring light for the correction sensor element. Accordingly a reflection of the projection light bundle may for example be split off and used as measuring light bundle without thereby causing any noticeable loss of light output of the projection light. The projection and the correction sensor device can therefore be operated with only one light source.
The projection optics may be designed so that it contains at least one intermediate image plane, and the correction sensor element may lie in the intermediate image plane or in a plane conjugated thereto. In this case the intensity distribution of the measuring light incident on the correction sensor element corresponds to that in the image plane, with the result that the determination of image defects can easily be performed. Moreover, in the region of the intermediate image plane a simple separation of the measuring light bundle from the projection light bundle can be performed since the projection light bundle is concentrated here, with the result that the measuring light bundle can be passed to the correction sensor device, while the projection light bundle is fully available for the projection.
A preferred embodiment comprises at least one optical decoupling element for decoupling the measuring light bundle from the projection light bundle in the region of an intermediate image plane and/or a plane conjugated there to. Such a decoupling element facilitates the separation of the at least one measuring light bundle from the projection light bundle.
The decoupling element may be a mirror. Decoupling mirrors may be produced in any suitable size, and in particular their size and optical strength may be matched exactly to the geometrical relationships for the decoupling of the measuring light bundle from the projection light bundle. Furthermore a decoupling mirror can be manufactured having a high surface quality of the reflecting surface, with the result that no additional image defects are produced.
The correction sensor device as well as the at least one correction manipulator may be designed so that they operate during the projection exposure. This increases the throughput of the projection exposure system since the projection exposure need not be interrupted for correction purposes.
The correction sensor device may comprise a wave front sensor. A determination of for example Seidel image defects may be carried out in a simple manner using such a sensor.
The position-sensitive correction sensor element may be a CCD array. A CCD array has a high position resolution and a high quantum efficiency.
The correction sensor device may include an adjustment manipulator for adjusting the correction sensor device relative to the projection optics. This permits the use of a pre-adjusted correction sensor device in conjunction with a plurality of projection objective lenses. This is particularly advantageous if the projection exposure system operates in a fixed cyclical mode and image defects of the projection optics that are to be corrected are always repeated within a cycle. In this case the correction sensor device need be coupled to the projection exposure system only during a first cycle in order to set adjustment values for the correction manipulator, while an adjustment program stored in the first cycle can be accessed in the subsequent cycles. In these subsequent cycles the correction sensor device may be used to adjust other projection exposure systems. For this purpose the correction sensor device is adjusted in each case with the aid of the adjustment manipulator.
An active mirror may be used as correction component. This mirror may comprise a plurality of mirror facets that maybe displaced independently of one another using correction manipulators or may also comprise a deformable reflecting surface. Finally, it is also possible to use an active lens. Such active components are described for example in U.S. Pat. No. 6,388,823, which is incorporated herein by reference, and are suitable for correcting image defects of any arbitrary symmetry.
Alternatively or additionally, a correction component may comprise a lens that is designed so that it can be displaced using the correction manipulator. More specifically, a lens may be provided that can be displaced in the direction of its optical axis. Alternatively or additionally a lens may be employed that can be displaced perpendicular to its optical axis. The use of such manipulable lenses for correcting various aberrations is known. Such a correction component is easy and inexpensive to fabricate.
A further object of the present invention is to develop a process of the type mentioned in the introduction in such a way as to ensure an efficient and at the same time image-corrected projection exposure.
This object is achieved according to the invention by a process having the following process steps:
a) providing at least one measuring light bundle guided through at least a part of the projection optics independently from the projection light bundle;
b) measuring of the optical properties of the measuring light bundle after the at least partial passage through the projection optics;
c) comparing the measured value with at least one predetermined desired value;
d) adjusting the measured optical property depending on the comparison with at least one correction component influencing the said optical property.
The advantages of this process according to the invention correspond to the advantages described above of the projection exposure system according to the invention.
The steps a) to d) may be repeated periodically during the projection exposure.
With such a process occurring image defeats can be corrected during the projection exposure without having to interrupt the projection exposure procedure. At the same time, since the at least one measuring light bundle is used to measure the image defects, the whole projection light bundle is available for the projection.
Preferably the wave front of the measuring light bundle is measured in an intermediate image plane or in a plane conjugated thereto. Measuring values that are simple to evaluate are accordingly available for occurring image defects.
An advantageous embodiment of the process comprises the following steps;
a) determining a deviation between a desired reflecting surface and an actual reflecting surface of a mirror of the projection optics, from the measured values;
b) calculating adjustment values for the actual reflecting surface;
c) shaping the actual reflecting surface corresponding to the calculated adjustment values.
The adjustment of a mirror performed according to the calculated correction ensures, depending on its position, a precise influencing of specific occurring image defects. Depending on whether the mirror is positioned close to the field or close to the aperture diaphragm, errors can be corrected that are selective or otherwise relative to various field points. Methods for influencing the shape of the reflecting surface of a mirror are known and comprise influencing rotationally symmetrical shapes as well as influencing shapes of arbitrary symmetry.
A further preferred embodiment of the process comprises the following steps:
a) determining a deviation between a desired position and an actual position of a displaceable lens of the projection objective lens;
b) calculating adjustment values for the actual position;
c) displacing the actual position corresponding to the calculated adjustment values.
Here too a predeterminable influencing of specific image defects is possible corresponding to the position of the displaceable lens. In this connection the lens may be chosen so that only a specific image defect is predominantly influenced, while other types of defect remain unaffected. Alternatively the displacement of the lens can also influence more than one image defect. Of course, several lenses may also be displaced, in which connection for example the displacement of the individual lenses is carried out in such a manner that specific image properties are changed in a predetermined magnitude, while changes in other image properties are compensated exactly by the lens displacements.
The adjustment of the optical property may be carried out while taking into account in addition expected image defects of optical components that influence the projection light bundle but not the measuring light bundle. The behavior of the optical components in that part of the projection optics not traversed by the measuring light bundle can be extrapolated from the number and type of the optical components through which the measuring light bundle passes. On the basis of this extrapolation the image defect of the projection optics can be influenced via the adjustment of the correction manipulator in such a way that not only are the image defects of the optical components through which the measuring light bundle passes corrected, but the overall result is that the projection optics is free of image defects. A further possibility is to load the projection optics with the respective lighting settings and then measure the image defects. In this way the image defects of the projection optics can be determined depending on the defects in the region traversed by the measuring light bundle.
The presetting of the desired value may be made depending on a lighting setting. Known lighting settings of this type have for example a homogeneous or also an annular lighting intensity in an aperture diaphragm plane of the projection optics. Lighting intensity distributions having multiple symmetry in the aperture diaphragm plane of the projection optics may also be employed. Depending on the geometry and symmetry of the lighting setting, a lighting-induced image defect of corresponding geometry and symmetry may result. With a known lighting setting an adjustment with the corresponding symmetry may therefore be effected on the basis of the measuring values of the correction sensor device.
The presetting of the desired value may be made depending on the type of object. The transmission of the object, for example a reticule in microlithography, may influence the resulting image defect in the projection optics. If the type of object is known, this may appropriately be taken into account in the correction of the image defects.